We propose to study, in open-chest, anesthetized dogs, some of the factors that affect the rates of release and reuptake of norepinephrine at the cardiac sympathetic terminals. We shall measure the changes in heart rate, myocardial contractile force, and coronary blood flow during and after a train of stimuli to the cardiac sympathetic nerves. These cardiac responses will be correlated with the changes in the rate of norepinephrine overflow into the coronary sinus blood. The relative roles played by the amine uptake processes and by diffusion of neurotransmitter into the coronary circulation will be compared at certain specific sites in the heart, notably in the ventricular and in the S-A nodal region. In different experiments, specific drugs will be employed to block the presynaptic alpha-receptors, the neuronal amine uptake process, or the extraneuronal amine uptake process. As examples of the roles played by the various mechanisms, we shall test several hypotheses to account for the observations that (1) the chronotropic responses to sympathetic stimulation are much more prolonged than are the inotropic responses, when the neuronal uptake mechanism is blocked, and (2) the responses to exogenously administered norepinephrine are potentiated after neuronal uptake blockade, whereas the responses to endogenously released norepinephrine are not potentiated. We shall also assess the efficacy of digitalis glycosides as neuronal uptake blocking agents and as potentiators of certain sympathetic-parasympathetic interactions in the hearts of intact animals.